This invention relates generally to circuit arrangements for detecting unrecorded zones of a phonograph record by means of a photodetector so that the movement of a tonearm carrying a stylus is controlled to position the stylus at a desired unrecorded zone to playback a selected music or information piece.
Various devices and circuits designed to detect unrecorded zones between consecutive music or information pieces for the above-mentioned purpose are known. In conventional devices, light emitted from a light source, such as a lamp or a light-emitting diode, is applied to a phonograph record so that light reflected at the surface thereof is received by a photodetector. Since the reflection coefficient at unrecorded zones is much higher than that of recorded zones or sound groove portions, the unrecorded zones can be detected by watching the intensity of reflected light. Namely, an output signal from the photodetector is processed to distinguish unrecorded zones from sound groove zones.
In conventional circuit arrangements, therefore, the output signal from such a photodetector is differentiated so that the amplitude and/or the rising slope of the differential pulse is detected. However, as will be described later in detail, conventional devices are apt to produce false detection signals each indicative of an unrecorded zone, or to overlook unrecorded zones for various reasons. As a result, the pickup stylus is undesirably lowered onto a sound groove zone in the case of a false detection signal is generated, and therefore, there is a chance that the sound groove zones become damaged. In the case that unrecorded zones are overlooked, positioning of the stylus onto a desired unrecorded zone cannot be expected. Furthermore, when accurate differentiation between sound groove zones and unrecorded zones is difficult, it would take a long time to select a desired piece of music or information.